The present invention relates to a secondary battery having a separator and more particularly to a cell stack zinc-halogen secondary battery.
The cell stack zinc-halogen secondary battery is composed of a stacking of the unit secondary cells, an electrolyte circulation system and the electrolyte storage tanks.
Each secondary unit cell is constructed of a negative electrode chamber and a positive electrode chamber divided by a separator and both chambers are filled with the electrolyte.
The electrolyte circulation system includes a negative electrolyte circulation system and a positive electrolyte circulation system. The negative electrolyte circulation system is arranged so that the negative electrolyte is exited from each negative electrode chamber of the unit secondary cells and collected, and then the negative electrolyte is returned to the negative electrode chambers through a negative electrolyte storage tank thereby circulating the negative electrolyte. The positive electrolyte circulation system is arranged so that in the like manner as the negative electrolyte circulation system, the positive electrolyte is exited from each positive electrode chamber of the unit secondary cells and collected, and then the positive electrolyte is again returned to the positive electrode chambers through a positive electrolyte storage tank thereby circulating the positive electrolyte.
The negative electrolyte circulation system includes a pump for circulating the negative electrolyte and the negative electrolyte storage tank for storing the negative electrolyte. Also, in the like manner as the negative electrolyte circulation system, the positive electrolyte circulation system includes a pump for circulating the positive electrolyte and the positive electrolyte storage tank for storing the positive electrolyte.
A plurality of projections are formed on the opposite sides of the separator for separating negative and positive electrodes thus forming a given space between the separator and the negative electrode and between the separator and the positive electrode, respectively. The separator consists of an ion permeable sheet which permeates only zinc ions.
The negative electrolyte consists of an aqueous solution of zinc bromide and the positive electrolyte consists of an aqueous solution of zinc bromide in which the bromine molecules are dissolved.
With this cell stack zinc-halogen secondary battery, the following oxidation-reduction reactions take place during the periods of charging and discharging.
More specifically, during the charging period the zinc ions in the negative electrode chamber and/or the positive electrode chamber are attracted to the negative electrode so that the zinc ions are furnished with electrons and reduced to metal zinc thus depositing as metal zinc on the surface of the negative electrode. In this case, the zinc ions in the positive electrode chamber are passed through the separator and attracted to the negative electrode. Also, the bromine ions in the positive electrode chamber are attracted to the positive electrode so that the bromine ions lose electrons and are oxidized into bromine molecules thus depositing as bromine molecules on the surface of the positive electrode. The bromine molecules oxidized and deposited on the surface of the positive electrode are dissolved into the positive electrolyte. The bromine molecules are not permeable through the separator and therefore the bromine molecules are retained in the positive electrolyte.
During the discharging period, the metal zinc on the negative electrode surface are oxidized and converted to the zinc ions so that the zinc ions enter the negative electrolyte while leaving the electrons at the negative electrode and thus a part of the zinc ions is passed through the separator into the positive electrolyte. On the other hand, the bromine molecules in the positive electrolyte are reduced and converted to bromine ions on the surface of the positive electrode.
Then, since the separator is formed with the plurality of projections on the opposite sides as mentioned previously, during the charging period the flow of zinc ions tends to concentrate around the projections in the negative electrode side and thus the localized formation of dendritic metal zinc tends to occur at the portions of the negative electrode surface which are opposite to the adjacent portions of the projections. Then, the dendritic metal zinc is low in mechanical strength and therefore there is the danger of its forward end breaking off thus deteriorating the ratio of the amount of current produced by the discharging to the amount of current required for the charging (hereinafter referred to as a current efficiency). Moreover, there is the danger of the dendritic metal zinc growing excessively and breaking through the separator thus causing the positive electrolyte to enter the negative electrode chamber which causes a self-discharge and thereby deteriorates the current efficiency. Also, there is the danger of the dendritic metal zinc breaking through the separator short-circuiting to the positive electrode and making the secondary battery inoperable.